Fix api-call in update_post

[3mb3dw0rk5]

According to Mattermost-API-v4 documentation https://api.mattermost.com/#tag/posts%2Fpaths%2F~1posts~1%7Bpost_id%7D~1patch%2Fput the update_post shall be called with sub-path /patch.

[attzonko]

Thanks for finding and fixing this. For future reference:

• Updating post with /posts/{post_id} only sets omitted fields will be treated as blank, which is undesirable for our update_post() API
• Patching a post with /posts/{post_id}/patch correctly updates only the field we wish to update leaving the rest intact